1. Field of the Invention
This invention relates to the improvement of a rolling bearing for supporting the rotating shaft, for example the pulley shaft, of a belt-type continuously variable transmission (CVT) for the wheels of an automobile or the like. More specifically, together with stabilizing the friction coefficient at the area of friction engagement between the belt and pulley, it makes possible construction that is capable of maintaining sufficient durability even when using a low-viscosity CVT fluid (combined use as ATF oil) in order for low fuel consumption.
2. Description of the Background Art
Various kinds of belt-type continuously variable transmissions have been considered in the past, such as disclosed in Japanese Patent Unexamined Publication No. H08-30526, as the transmission unit of an automatic transmission for an automobile, and some are actually in use. FIG. 1 shows the basic construction of this kind of belt-type continuously variable transmission. The belt-type continuously variable transmission comprises a rotating shaft 1 on the input side and a rotating shaft 2 on the output side and they are arranged such that they are parallel with each other. These rotating shafts 1, 2 are each rotatably supported by a pair of rolling bearings 3 inside the transmission casing (not shown in the figure).
As shown in detail in FIG. 2, each of these rolling bearings 3 has an outer race 4 and inner race 5 that are concentric with each other. Of these, the outer race 4 has an outer raceway 6 formed around its peripheral inner surface, and the inner race 5 has an inner raceway 7 formed around its outer peripheral surface. A plurality of rolling elements 8 are located between the outer raceway 6 and inner raceway 7, and they are supported by a retainer 9 such that they can roll freely. The outer race 4 of each of the rolling bearings 3 that are constructed in this way, is fitted and fastened into part of the transmission casing, and the inner race 5 is fitted and fastened onto the rotating shaft 1 on the input side or onto the rotating shaft 2 on the output side. Also, with this construction, both of the rotating shafts 1, 2 are supported on the inside of the transmission casing such that they rotate freely. Conventionally, a bearing having an outer race 4, inner race 5 and rolling elements 8 that were made of SUJ2 bearing steel were used as the rolling bearing 3.
Of the rotating shafts 1, 2, the rotating shaft 1 on the input side is rotated by a driving source 10 such as an engine by way of the starting clutch 18 of a torque converter or solenoid clutch. Also, a drive-side pulley 11 is located in the section that is located between the pair of rolling bearings 3 in the middle section of the rotating shaft 1 on the input side, and this drive-side pulley 11 and the rotating shaft 1 on the input side are rotated in synchronization. The space between the pair of drive-side pulley plates 12a, 12b of the drive-side pulley 11 can be freely adjusted by using the actuator 13 on the drive side to move the drive-side pulley plate 12a (the left one in FIG. 1) in the axial direction. In other words, the groove width of the drive-side pulley 11 can be freely widened or narrowed by the drive-side actuator 13.
On the other hand, a follower-side pulley 14 is located in the section between the pair of rolling bearings 3 in the middle section of the rotating shaft 2 on the output side, and this follower-side pulley 14 rotates in synchronization with the rotating shaft 2 on the output side. The space between the pair of follower-side pulley plates 15a, 15b of this follower-side pulley 14 can be freely adjusted by moving the follower-side pulley plate 15a (the right one in FIG. 1) in the axial direction by the follower-side actuator 16. In other words, the groove width of the follower-side pulley 14 can be freely widened or narrowed by the follower-side actuator 16. Also, an endless belt 17 runs around this follower-side pulley 14 and the drive-side pulley 11. A metal belt is used for this endless belt 17.
In the belt-type continuously variable transmission constructed as described above, the power transmitted from the driving source 10 to the rotating shaft 1 on the input side by way of the starting clutch 18, is transmitted from the drive-side pulley 11 to the follower-side pulley 14 by way of the endless belt 17. A belt that transmits power in the pushed direction and a belt that transmits power in the pulled direction are known as this kind of endless belt 17. In either case, the power that is transmitted to the follower-side pulley 14 is transmitted from the rotating shaft 2 on the output side to the drive shaft 21 by way of a reduction gear train 19 and differential gear 20. When the transmission gear ratio between the rotating shaft 1 on the input side and the rotating shaft 2 on the output side is changed, the groove widths of both pulleys 11 are widened or narrowed in relation to each other.
For example, when the deceleration ratio between the rotating shaft 1 on the input side and the rotating shaft 2 on the output side is increased, the groove width of the drive-side pulley 11 increases, and also the groove width of the follower-side pulley 14 decreases. As a result, in the sections where the endless belt 17 runs around of both pulleys 11, 14, the diameter in the section of the drive-side pulley 11 becomes smaller and the diameter in the section of the follower-side pulley 14 becomes larger, and deceleration occurs between the rotating shaft 1 on the input side and the rotating shaft 2 on the output side. On the other hand, when the acceleration ratio between the rotating shaft 1 on the input side and the rotating shaft 2 on the output side is increased (the deceleration ratio is decreased), the groove width of the drive-side pulley 11 decreases and the groove width of the follower-side pulley 14 increases. As a result, in the sections where the endless belt 17 runs around of both pulleys 11, 14, the diameter in the section of the drive-side pulley 11 becomes larger and the diameter in the section of the follower-side pulley 14 becomes smaller, and acceleration occurs between the rotating shaft 1 on the input side and the rotating shaft 2 on the output side.
When the belt-type continuously variable transmission that is constructed and function as described above is operating, lubrication oil is supplied to all of the moving parts to lubricate these moving parts. CVT fluid (combined use as ATF oil) is used as the lubrication oil for the belt-type continuously variable transmission. The reason for this is to increase and stabilize the friction coefficient at the area of friction engagement between the metal endless belt 17 and the drive and follower-side pulleys 11, 14. Also, the CVT fluid is circulated at the area of friction at a flow rate of 300 cc/min or more, to lubricate this area of friction. Moreover, pat of the CVT fluid passes through the inside of the rolling bearings 3 (for example at a flow rate of 20 cc/min or more) to lubricate the area of rolling contact of the rolling bearings 3. Therefore, there is a good possibility that foreign matter such as abrasive material that is generated due to the friction between the metal endless belt 17 and the pulleys 11, 14, or gear powder that is generated due to friction in the reduction gear train 19, will get into and become mixed with the CVT fluid. This foreign matter causes damage to the rolling contact areas in the rolling bearings 3 and causes a drop in the durability of the bearings. Therefore, conventionally, by making the size of the rolling bearings 3 larger, or by increasing the diameter (ball diameter) of the rolling elements 8, the basic dynamic load rating of the rolling bearings 3 was increased to give extra life to these rolling bearings 3.
The frequency (f: Hz) of vibration in the belt constructed as described above is expressed using the number of friction pieces (Zb) and the rpm of the belt (Nb: rpm) as “f=Zb×(Nb/60)”. Normally, the number of friction pieces is 250 to 400 pieces, and in this case, when the rpm of the engine changes from 600 rpm to 7,000 rpm, the first component of the frequency of the vibration that occurs in the primary pulley is 1,000 Hz to 3,000 Hz during deceleration, and 10,000 Hz to 35,000 Hz during acceleration.
This frequency is higher than the frequency of the vibration that occurs when the gears mesh in a manual transmission (MT) or in a normal automatic transmission (AT) (except for the continuously variable transmission). This is thought to be because the number of friction pieces for a belt-type CVT is 250 to 400, and is more than the number, specifically 50 or less of gear teeth that mesh in the case of a MT and AT.
Also, when the automobile is in operation, acceleration and deceleration are repeated, so there may be resonation of the belt vibration and body vibration. Moreover, vibration at various frequencies occurs in the body, and thus it becomes easy for resonance to occur frequently with the belt. As a result, it becomes easy for large vibration to occur in the rolling bearings for the belt-type CVT.
On the other hand, for a belt-type CVT, it is desired that the power transmission efficiency of the belt be improved, noise of the belt drive be controlled, and that friction between the pulleys and the belt be suppressed, and from these aspects, it is preferred that lubrication oil having high fluidity (low viscosity) be used.
In recent years, with the object of maintaining the efficiency of the belt-type continuously variable transmission, keeping noise that is generated during operation to a minimum, and suppressing friction between the drive-side pulley 11 and follower-side pulley 14 and the endless belt 17, the use of CTV fluid having an even lower viscosity has been considered as the lubrication oil. In that case, when using standard bearings as the rolling bearings 3 for supporting the rotating shaft 1 on the input side and the rotating shaft 2 on the output side, it is thought that there is a larger possibility of premature flaking due to insufficient oil layer formation rather than flaking that starts at the point of indentation due to foreign matter mixed in the oil. In other words, since the rolling bearings for supporting the rotating shaft of the pulley are located on the side surfaces of the pulley, it is difficult for lubricant to be supplied, and when CVT fluid having a low viscosity is used, there would be larger possibility that the state of the oil layer formed at the areas of rolling contact between the outer raceway 6 and inner raceway 7 and the rolling elements 8 will be insufficient due to the vibration acting in the radial direction and axial direction as the belt varies. Also, it is thought that the possibility of premature flaking due to slippage will increase at the areas of rolling contact.
In other words, in the case of rolling bearings 3 comprising an outer race 4, inner race 5 and rolling elements 8 that are made from typical bearing steel such as SUJ2, it is thought that the possibility of premature flaking due to slippage will increase when CVT fluid having a low viscosity is used in which the kinematic viscosity of the base oil is 40 mm2/sec or lower (40×10−6 m2/sec or lower) at 40, and is 10 mm2/sec or lower at 100. In that case, when the temperature of the rolling bearings 3 during operation of the belt-type continuously variable transmission exceeds 100, the viscosity of the CVT fluid penetrating in the rolling bearing 3 for lubrication on the rolling contact area becomes a very low value of 10 mm2/sec or less. As a result, the strength of the oil film that exists at the area of rolling contact decreases, and it becomes easy for the oil film at the area of rolling contact to be broken up due to effects such as differential motion, revolution, or spinning. Also, when the oil film is broken up, metallic contact occurs at this area of rolling contact, which promotes fatigue of the surface layer and causes premature flaking to occur. Of course, by increasing the basic dynamic load rating of the rolling bearings 3 and giving extra life to the rolling bearings 3, it is possible to maintain the required durability, however, since the weight due to increasing the size of the bearing increases and the rolling resistance increases, it is not desirable. Also, by increasing the flow rate of the CVT fluid that passes through the rolling bearings 3, it is possible to prevent the oil film from breaking up, and it is also possible to improve durability. However, this method causes a drop in the efficiency of the overall belt-type continuously variable transmission due to the increase of pump loss due to the circulating large amounts of CVT fluid, which is not desirable.
Incidentally, a substantial portion of the loading applied to the rolling bearings 3 installed in the belt-type continuously variable transmission is radial loading that is applied from the endless belt 17, and the direction of this radial loading is always constant. Also, of the rolling contact area of the rolling bearing 3, the inner raceway 7 and the rolling contact surface of the rolling elements 8 rotates, but the outer raceway 6 does not rotate. Accordingly, fatigue of the surface layer advances the most in a specific section of the outer raceway 6 (section that supports the radial loading). In other words, the outer raceway 6 is subjected to the most severe condition with respect to the rolling fatigue life. Therefore, maintaining the rolling fatigue life of the outer raceway 6 is important from the aspect of maintaining the overall durability of the rolling bearing 3.
For example, FIGS. 3(A) and 3(B) show the fatigue level at the area of rolling contact when using CVT fluid with a low viscosity as the lubrication oil, which are the results found through fatigue analysis for a typical gear-type transmission and belt-type continuously variable transmission. The fatigue analysis is a measurement method disclosed in Japanese Patent Examined Publication No. S63-34423 of measuring fatigue level due to rolling contact fatigue, and the fatigue level is expressed as F=B+KR (where B is the decrease in the half value width of X-ray diffraction in the Martensitic-phase; K is a constant that differs according to the material, and R is the amount of decrease of retained austenite). In other words, in order to find the fatigue level F, the half value width of X-ray diffraction in the Martensitic-phase before and after rolling fatigue at the area of rolling contact between the metal materials, and the amount (volume %) of retained austenite are measured. Moreover, the constant set for the type of material is taken to be K, and the difference between the amount of retained austenite before the fatigued state and the amount of retained austenite in the fatigued state is taken to be .R. Also, the difference in the half value width of X-ray diffraction in the Martensitic-phase before the fatigued state and the half value width of X-ray diffraction in the Martensitic-phase in the fatigued state is taken to be .B. The fatigue level F is then found by substituting the reduced amount .B of the half value width of X-ray diffraction in the Martensitic-phase, and the reduced amount .R of the retained austenite into the equation F=K..R+.B. Also, this fatigue level F is correlated and evaluated with reference values that are created beforehand and that correspond to each location of rolling contact, to measure the fatigue level at each of these locations.
In the results of the fatigue analysis performed under these conditions shown in FIGS. 3(A) and 3(B), FIG. 3(A) shows the fatigue level of the outer raceway of a rolling bearing that is installed in a typical gear-type transmission, and FIG. 3(B) shows the fatigue level of the outer raceway of a rolling bearing installed in a belt-type continuously variable transmission. The higher the value of the fatigue level is the more the fatigue progresses, which indicates that the flaking life becomes short. The fatigue level shown in FIG. 3(A) for the surface of the outer raceway of the rolling bearing installed in a typical gear-type transmission was 1.4, while the fatigue level shown in FIG. 3(B) for the same part in a belt-type continuously variable transmission was 2.8, or twice as high.
As can be clearly seen from FIG. 3(A) and FIG. 3(B), when CVT fluid having a low viscosity is used as the lubrication oil for a belt-type continuously variable transmission, it becomes easy for premature flaking to occur in the outer raceway of the rolling bearings of the rotation-support unit of the belt-type continuously variable transmission.
Taking the above conditions into consideration, in order to obtain a belt-type continuously variable transmission having excellent transmission efficiency and sufficient durability, this invention was made to provide a rolling bearing for a belt-type continuously variable transmission in which it is difficult for damage such as premature flaking to occur in the outer raceway 6 of the rolling bearing 3 for supporting the pulley such that it rotates freely, even when CVT fluid having a low viscosity is used as the lubrication oil.